After creating a Windows Boot Camp partition on a Mac, you might eventually run out of space and want to make it larger. Here’s how to do it without reinstalling Windows. ***Remember to back up first, because a mistake here could erase all data on your machine!***

Step 2. In OS X, shrink the size of your Mac boot partition using the Disk Utility GUI, and reboot.

Step 3. OS X will automatically format the new unused space as HFS+, and we want this space available for Windows, so go to the Terminal and type:
diskutil list

Find the partition you want to erase—probably disk0s4, but double-check that the size is correct—and then type:
sudo diskutil eraseVolume Free\ Space null disk0s4

Step 4. Right now your Windows partition will be temporarily rendered unbootable. To fix it, while still in OS X, download and install the gdisk utility.

Step 5. Type:
sudo gdisk /dev/disk0

Step 6.***If you get any error messages at this point, stop!*** Otherwise, carefully enter the following (adapted from this forum thread):
r <enter> go to the recovery & transformation menu
h <enter> create a new hybrid MBR
5 <enter> add partion 5 to the MBR
y <enter> set up EFI boot
<enter> accept the default MBR hex code of 07
y <enter> set the bootable flag
n <enter> do not protects more partitions
o <enter> review (but don't apply) the changes

Step 7. If this went according to plan, you should see two partitions: one type EE and one 07, with the 07 entry marked with * under Boot. If this is what you see, apply the changes by typing:
w <enter> write partition table to disk

Step 8. Now reboot into Windows and install the free version of Partition Wizard.